Abstract

Decisions must often be made in the face of uncertain evidence. It has been proposed that the brain utilizes its neuromodulatory brainstem systems to broadcast uncertainty signals across the brain, thereby triggering changes in global brain state and neural plasticity. Recent animal work indicates that task-evoked responses in neuromodulatory systems signal two computational variables, at different times during perceptual decisions: decision confidence (i.e., the complement of uncertainty) before feedback and prediction errors (i.e., deviations from expected reward) after feedback. Neuromodulatory systems also regulate central arousal state. We here monitored pupil diameter, a marker of central arousal state, while human subjects performed a challenging perceptual choice task with delayed monetary reward. We quantified evoked pupil responses during decision formation and after reward feedback. During both intervals, decision difficulty and accuracy had interacting effects on pupil responses. This interaction reflected decision uncertainty prior to feedback and prediction error after feedback. Critically, the pattern of pupil responses during both intervals was in line with a model that uses the decision-maker's graded belief in choice accuracy to anticipate rewards and compute prediction errors. We conclude that pupil-linked arousal systems are modulated by internal belief states, in line with recent insights into dopamine signaling.